Quickstart needs `dart pub get` instructions

[grant]

The README need more detailed instructions for getting started with Dart for someone who has never used dart.

https://github.com/GoogleCloudPlatform/functions-framework-dart/blob/main/docs/quickstarts/01-quickstart-dart.md

dart test
No .dart_tool/package_config.json file found, please run 'dart pub get'.

Usage: dart test [files or directories...]

Looks like you need to run dart pub get before running the quickstarts.